One hundred and sixteen knees of 33 patients with patellar dislocation and 25 normal children 12-18 years of age were examined by ultrasonography to measure the cartilaginous sulcus angle on the patellar surface of the femur. In knees with patellar dislocation, the cartilaginous sulcus angle measured between 154 and 195 degrees, exceeding the corresponding values of normal knees (134-153 degrees). In patient knees, the cartilaginous sulcus was also consistently wider than the underlying osseous sulcus. This suggests that in pediatric patients with patellar dislocation, the actual patella-stabilizing ability of the femoral sulcus is weaker than the osseous outline in axial radiographs would lead one to suppose. It would appear also that by measuring the cartilaginous sulcus angle of the femur, a clear distinction can be made between normal knee joints and joints displaying patellar instability.